When a rod is stretched beyond its yield point, it permanently deforms and doesn't regain its original length.
The stresses created during loading are not completely removed even after unloading due to plastic deformations. These remaining stresses are known as residual stresses.
Consider a small plug welded to a large plate. During welding, the plug's temperature increases significantly, reducing both the modulus of elasticity and stress.
Once the plug cools down after welding, it causes an increase in the modulus of elasticity, and yielding happens at a fairly constant stress level, creating residual stress in the plug and the weld.
The cooling of cast or hot-rolled metals also causes residual stresses. In these cases, the outer layers cool faster than the inner core, causing them to regain their stiffness more quickly.
On returning to room temperature, the inner core gets contracted more than the outer layers, resulting in residual longitudinal tensile stresses at the inner core.
The outer layers develop residual compressive stresses. Large residual stresses are removed through reheating and slow cooling.
18.20:
Residual Stresses
Residual stresses reside in a structure even after removing the original stress inducer. This phenomenon often arises from varied plastic deformations across different parts of a structure. Consider a rod stretched beyond its yield point. It will not regain its original length due to permanent deformation. Even after load removal, the rod does not entirely lose stress because of uneven plastic deformations, resulting in residual stresses. The computation of these stresses in structures is intricate. Factors like temperature changes during welding can cause plastic deformations, leading to a reduction in elasticity and the generation of residual stresses.
For instance, a plug welded to a plate heats significantly during the process, reducing its stress. As it cools post-welding, its elasticity increases, causing yielding at a constant stress level. This results in residual stress close to the steel's yield strength. Residual stresses also occur during the cooling of metals after casting or hot rolling. The outer layers cool faster than the inner core, regaining stiffness quicker and leading to tensile residual stresses in the core and compressive in the outer layers. These stresses can be significant, sometimes requiring removal through specimen reheating and slow cooling. Understanding and managing residual stresses is crucial in engineering design and manufacturing processes, ensuring the reliability and durability of structures and components.